In general we are concerned with how isolated cell membranes (in vesicle form) are able to accomplish and regulate the transport of precursors required for synthesis of cellular macromolecules, particularly nucleic acids. Though mammalian cells are capable of synthesizing their own precursors, an awareness of the extent of, and requirement for, reutilization of performed nucleic acid precursors, is emerging. Furthermore, the differences between growing and non-growing cells in uptake of nucleic acid precursors is more pronounced than for other metabolites. We feel that studying transport in isolated membrane vesicles is important because it attempts to use the simplest model in which the key function remains intact. In specific we are addressing the question of whether growth in normal and transformed mammalian cells in culture is controlled by alterations in specific membrane transport of metabolites, particularly nucleic acid precursors. Purified plasma membrane vesicles capable of concentrative uptake of purines, pyrimidines, and nucleosides are employed in order to distinguish biochemical mechanisms involved in transport from other cellular activities which influence uptake. We are focusing on characterization of the kinetics, mechanism, regulation, energy and lipid requirements of transport of purine and pyrimidine bases and nucleosides in highly purified membrane vesicles prepared from several cell lines and growth conditions.